1. Field of the Invention
The present invention relates to a fluorene derivative, and to an organic electroluminescence device (hereinafter, sometimes referred to as “organic EL device”) using the same. In particular, the present invention relates to an organic EL device having highly stable emission efficiency, which is obtained by using a fluorene derivative as a host in a light-emitting layer.
2. Description of the Related Art
In an old example of an organic EL device, a voltage has been applied to an anthracene evaporated film to emit light (see, Thin Solid Films, 94 (1982), 171). In recent years, however, applied research has been vigorously conducted on a transformation of an organic EL device as a light-emitting device having high-speed response and high efficiency into a device, including developments of materials for the device. This is because the device has such advantages that an area of the organic EL device can be increased more easily than that of an inorganic light-emitting device, the device provides desired color development through the developments of various new materials, and the device can be driven at a low voltage.
For example, as described in Macromol. Symp. 125, 1 to 48 (1997), an organic EL device generally includes a transparent substrate, two upper and lower electrodes formed on the transparent substrate, and an organic material layer including a light-emitting layer, the organic material layer being interposed between the two electrodes.
In recent years, investigation has been made into a device utilizing not only conventional light emission utilizing fluorescence upon transition from an excited singlet state to a ground state but also phosphorescence via a triplet exciton which is represented by technologies described in each of “Improved energy transfer in electrophosphoresent device”, D. F. O'Brien et al., Applied Physics Letters, Vol 74, No 3, p 422 (1999) and “Very high-efficiency green organic light-emitting devices based on electrophosphorescence”, M. A. Baldo et al., Applied Physics Letters, Vol 75, No 1, p 4 (1999). In each of those documents, an organic layer having a four-layer structure has been mainly used. The organic layer includes a hole-transporting layer, a light-emitting layer, an exciton diffusion-prevention layer, and an electron-transporting layer from an anode side. Materials used are a carrier-transporting material and a phosphorescent material Ir(ppy)3 shown below.

Further, as host materials of a triplet luminescent material typified by those described in Japanese Patent Application Laid-Open No. 2003-142267 and Japanese Patent Application Laid-Open No. 2004-273128, developments of host materials having high lowest excited triplet states (T1) have been conducted.
A variety of light ranging from ultraviolet light to infrared light can be emitted by changing the kind of a fluorescent organic compound. In recent years, active research has been conducted on various compounds.
In addition to an organic light-emitting device using any one of the low-molecular-weight materials as described above, an organic light-emitting device using a conjugate polymer has been reported by the group of the University of Cambridge (see, Nature, 347, 539 (1990)). In this report, there has been observed light emission from a single layer by forming polyphenylenevinylene (PPV) into a film by means of a coating system.
As described above, an organic light-emitting device has recently showed significant progress. The organic light-emitting device is characterized in that the organic light-emitting device can be transformed into a high-speed response, thin, and lightweight light-emitting device which can be driven at a low applied voltage and has high luminance and a variety of emission wavelengths. The characteristic suggests the potential of the device to find use in a wide variety of applications.
However, at present, output of light having additionally higher luminance, or additionally higher conversion efficiency is required. In addition, there still remain a large number of problems in terms of durability such as a change with time due to long-term use and deterioration due to an atmospheric gas containing oxygen or due to moisture. Further, although luminescence of blue, green, and red which have good color purity are necessary when the organic light-emitting device is applied to a full-color display and the like, the light-emitting device has not overcome the problem.
In addition, a large number of aromatic compounds and condensed polycyclic aromatic compounds have been investigated for their potential as a fluorescent organic compound to be used in an electron-transporting layer, a light-emitting layer, and the like. However, there is not yielded one which has sufficient luminance and durability.
Japanese Patent Application Laid-Open No. 2003-261471 describes application of a compound having a substituent at a position 4 of fluorene, which is related to the present invention, to an organic EL device.